In the photographic industry, webs of paper and film have for some time been simultaneously coated with several layers.
Multilayer coating processes of the type commonly used in the photographic industry include the so-called cascade coating process in which one or more liquid coating materials simultaneously flow down an inclined sliding surface and are coated on the moving web through a narrow gap (0.15-0.3 mm) between the edge of the coater and the web. In addition, the so-called curtain coating process has recently been gaining significance in the photographic industry. There are two versions of the curtain coating process, namely the so-called slot coaters and the so-called nozzle coater or sliding surface coater operating on the same principle as the cascade coater. In slot coaters, the coating material issues at the lower end of an outflow gap arranged transversely above the web to be coated where it directly forms a free-falling curtain. By contrast, in nozzle coaters or sliding-surface coaters, the coating material is forced through an outlet slot onto an inclined sliding surface, flows under the force of gravity down this sliding surface, which is arcuate or nozzle-shaped at its lower end, and only forms the free-falling curtain at the lower end of the nozzle, i.e. on leaving the nozzle. In both cases, the liquid curtain may consist of one or more layers.
The cascade coating process is limited in its scope of application and is attended by a number of disadvantages. Neither is it suitable for coating individual objects because their edges would disturb the coating process.
Thus, the gap between the edge of the coater and the web has to be kept relatively narrow (0.15 to 0.3 mm), so that, on the passage of a joint or if the edges of the moving web are slightly frayed, the web as a whole is torn. In addition, particles of dust can easily be left behind in the critical region of the coating edge, giving rise to so-called pencil lines in the layer applied.
In addition, on the passage of joints, so-called coating breaks can occur on account of the above-mentioned narrow gap. Furthermore, the coating material leaving the coater forms a meniscus at the inlet end of the web which is drawn more or less deeply into the gap between the edge of the coater and the web, depending on the vacuum applied to assist the coating process. The coating material is not replaced in this meniscus at the edge of the coater, with the result that partial hardening or thickening (through evaporation of the volatile constituents) occurs there. This also leads to the well-known pencil lines. Similar consequences arise from an irregular wetting line. Finally, the coating speed in this conventional process has an upper limit which depends on the number of layers, the quantities applied and the viscosities of the coating materials.
The above-mentioned conventional curtain coating process using slot coaters or nozzle coaters has been described in various forms and with various apparatus which show that it is possible in this way to produce single-layer and multiple-layer coatings at high speed. However, these processes and apparatus are also attended by serious disadvantages.
Thus, the curtain coating system using a slot coater is limited to two-layer coatings. When filled with the coating material, the slot coater is difficult to vent so that small bubbles give rise to faults in the layer applied during production.
In both the nozzle coater and the slot coater, disturbances attributable to unequal wetting of the discharge lips, deposits of dust and partial hardening and thickening of the coating material originate at the discharge lip at which the curtain is formed (in the same way as the coating edge of cascade coaters). In practice, this leads to faults such as pencil lines, breaks in the curtain, etc. In the event of disturbances such as these, production has to be stopped and the discharge lips have to be cleaned and re-aligned. Thus, in addition to the loss of production time, considerable losses of material occur in high-speed multi-layer coating.
In addition, where the nozzle coater is used, the two surfaces of the curtain have different surface tensions because one surface, namely the layer remote from the sliding surface, is always in contact with the atmosphere for a longer period. Since the surface forces are unequal, the curtain can become unstable. The surface-active additives introduced into the coating material also act differently, according to the medium with which they are in contact. In the case of slot coaters, both surfaces are simultaneously exposed to the atmosphere during formation of the curtain. In nozzle coaters, one surface is in contact with the atmosphere for a longer period than the other surface.
German Offenlegungsschrift No. 2,351,369 (corresponding to U.S. Pat. No. 3,973,062) describes a coating system which is based on a combination of two nozzle coaters. Each of the two nozzle coaters produces up to two liquid films which, together with the liquid films produced by the other nozzle coater, form a curtain which drops vertically onto the web to be coated. In addition to the two nozzle coaters, this known coating system contains a third component consisting of a prismatic body of which the cross-section is in the form of an equilateral triangle. One longitudinal edge of the prismatic body, the so-called discharge edge, lies above the gap formed by the lips of the two nozzle coaters. One layer of liquid coating material flows over each of the two surfaces forming the discharge edge of the prismatic body. At the discharge edge, the two layers form a double layer which drops as a curtain between the layers supplied by the two nozzle coaters and combines to form a multiple layer which then flows down onto the web to be coated in the form of a free falling curtain.
In this case, a multiple layer is applied by means of a relatively complicated system. The coating system has the same disadvantages as the known nozzle coaters. In addition, three coaters are required for only five layers. They are very difficult and expensive to operate. Owing to the intermediate application of two layers in two layers, there is a considerable danger of air being trapped inside the layers. The faults mentioned in reference to nozzle coaters, such as pencil lines caused by deposits of dust, evaporation residues and irregular wetting lines, occur on the front and back of the curtain.
An object of the present invention is to provide a simplified process and an apparatus for simultaneously applying several layers of liquid coating materials to objects, particularly webs acting as layer supports, which enables a simple and compact structure to be obtained and which avoids the above-mentioned disadvantages of conventional processes and apparatus.